Balloon angioplasty is a nonsurgical method of clearing coronary and other arteries, blocked by atherosclerotic plaque, fibrous and fatty deposits on the walls of arteries. A catheter with a balloon-like tip is threaded up from the arm or groin through the artery until it reaches the blocked area. The balloon is then inflated, flattening the plaque and increasing the diameter of the blood vessel opening. The arterial passage is thus widened. The benefits of angioplasty of arteries have been amply demonstrated.
There are limitations, however, to this technique's application, depending on the extent of the disease, the blood flow through the artery, and the part of the anatomy and the particular vessels involved. Plaque build-up and/or severe re-stenosis recur at about 40-50 percent of those treated. A huge number of patients experiencing a successful primary angioplasty procedure are destined to require a repeat procedure. Balloon angioplasty can only be characterized as a moderate-success procedure.
Recently, a newer technique of inserting a metallic stent is used to permanently maintain the walls of the vessel treated at its extended opening state. Vascular stents are typically composed of a biocompatible material such as a biocompatible metal wire of tubular shape, a metallic perforated tube, or a tiny mesh tube used by heart surgeons to prop open the weak inner walls of diseased arteries. The stent should be of sufficient strength and rigidity to maintain its shape after deployment, and to resist the elastic recoil of the artery that occurs after the vessel wall has been stretched. The stents are often used in conjunction with balloon angioplasty to prevent restenosis after the clogged arteries are treated. Despite of its considerable benefits, coronary stenting alone is not a panacea, as studies have shown that about 30% of the patient population subjected to that procedure will still experience restenosis. Risks of inflammation of the vessel walls are exacerbated by the presence of the stent.
When a clogged artery is widened, the plaque is broken up and the underlying collagen or damaged endothelium is exposed to the blood flow. Collagen has a prothrombotic property which is part of a body healing process. Unless collagen or the damaged endothelium is passivated, treated, or modulated, the chance for blood vessel clotting as well as restenosis still exists. Several U.S. patents disclose incorporation of drugs or radioactive elements onto the stent for slow release into the blood stream. One example is U.S. Pat. No. 5,843,163 to Wall who discloses an expandable stent having radioactive treatment means. Another example is U.S. Pat. No. 5,882,291 to Bradshaw et al. who discloses a device and methods for controlling dose rate during intravascular radiotherapy. U.S. Pat. No. 5,871,436 to Eury discloses radiation therapy method by immersing the device in a solution of the radioisotope just prior to device implantation. However, none of the above-mentioned patents disclose releasing therapeutic agents into the tissue, instead to the blood stream, at the stent contact site.
A metallic stent is generally coated with a biodegradable or non-degradable coating which incorporates a radioactive source. One particular example is U.S. Pat. No. 5,871,437 to Alt who discloses a radioactive stent for treating blood vessels to prevent restenosis. Said patent discloses that not only the restenosis triggered by the proliferation of smooth muscle cells is inhibited by a radioactive material, but the restenosis triggered by thrombus formation is also inhibited, by incorporating into the coating carrier hirudin, iloprost or other anti-coagulant. However, due to continuous blood flow to wash away the active ingredient inside the coating layer from the coated stent, the anti-restenosis effect diminishes quickly or compromised greatly. There is a clinical need to diffuse or embed the active ingredient, such as radioactive elements, anti-coagulant and the like, into the lesion site where the stent contacts the tissue of the arterial wall. An appropriate thermal energy plays a critical role in enhancing the diffusion process of an active ingredient into the tissue wall.
Radiofrequency therapeutic protocol has been proven to be highly effective when used by electrophysiologists for the treatment of tachycardia; by neurosurgeons for the treatment of Parkinson's disease; and by neurosurgeons and anesthetists for other RF procedures such as Gasserian ganglionectomy for trigeminal neuralgia and percutaneous cervical cordotomy for intractable pains. A stent deployed within a vessel, such as a coronary stent, has excellent metal-to-tissue contact surface. It becomes an ideal medium for applying thermal energy to the tissue needed for impregnation or embedding of active ingredients, such as radioactive substances or anti-coagulants. Radiofrequency protocol, which exposes a patient to minimal side effects and risks, is generally applied precisely to the stent-to-tissue contact site to obtain the desired thermal effect to accelerate the diffusion and embedding of an active ingredient into the local stent-contact tissue for prolonged therapeutic effects.